Analysis of the mechanism of regulaton of the nitrate reductase activity in the cyanobacterium Synchococcus elongatus Strain PCC 7942

Abstract

In Synchococcus elongatus, nitrate reductase (NR) is inhibited upon addition of NH₄⁺ to the medium. The inhibition involves the PII protein, but little is known about the detail of the regulatory mechanism. Since NR was recently found to interact with the DNA-binding domain of NblR, the response regulater that responds to high light and nitrogen/sulfur deficiency, we characterized an nblR mutant in this study to determine whether or not NblR regulates NR. Since NH₄⁺ inhibits the nitrate-nitrite transpoter (NRT) as well as NR in the wild-type cells, we constructed the nblR mutant from NC2, a mutant having NH₄⁺-resistant NRT activity, and examined the effects of NH₄⁺ thereon. The mutant showed NH₄⁺-responsive complete inhibition of NO₃^- uptake, indicating that NR is tightly regulated. Thus, NblR was shown to be clearly not essential for the regulation of NR.